1. Field of the Invention
The invention concerns X-ray scanners and, more particularly, a method enabling the correction, in the the images obtained, of the defects due to the movements of the measurement set. It also concerns a system for the correction of defects such as this.
2. Description of the Prior Art
To examine patients, increasing use is being made of X-ray machines called "scanners" which take cross-sectional views of a patient. These machines are based on the physical phenomenon of the absorption of X-rays by the human body. This absorption is directly related to the distance x travelled by X-rays in the body, according to the formula: EQU I=I.sub.o e.sup.-bx
wherein:
I.sub.o is the intensity of the radiation entering the human body,
I is the intensity of radiation leaving the human body,
b is a coefficient of attenuation that depends on the body crossed. In a logarithmic scale of measurement, the attenuation I/I.sub.o is equal to bx, i.e. it is proportional to the distance x. These machines consist essentially, as shown in figure 1, of an X-ray source 10 associated with a detection device 11, these two elements being arranged with respect to each other in a fixed geometrical relationship so that the body to be examined can be interposed between them. Furthermore, they are supported by a structure (not shown) which can rotate around the body to be examined so as to irradiate the body along different angles. The X-ray source, which is controlled by a device 13, emits rays according to an angular sector with a width that is sufficient to illuminate the entire cross-section of the body. The detection device 11 is shaped like an annular sector, the length of this sector being adapted to the width of the X-ray beam and being formed by a large number of elementary detectors 12, juxtaposed beside one another.
To obtain an image of the cross-section of the human body through which the X-ray beam travels, the supporting structure of the source 10 and of the detection device 11 is made to rotate on the body, and the output signals of the elementary detectors 12 are measured so as to process them appropriately according to known methods in order to draw therefrom an image representing the cross-section. For this processing operation, the elementary detectors 12, also called channels, are connected to an electronic device 14 which first of all computes the logarithm of the signals received so as to obtain a signal, the amplitude of which is proportional to the attenuation of the X-rays. The axis of rotation of the structure formed by the source 10 and the detection device 11 is located at the point 0, which means that the source 10 and the detectors 12, each describes during the rotation of the structure, a circumference having a center 0 and a known radius. This is true only in theory, for the manufacturing and using tolerances lead to deviations from the theoretical positions. The result thereof is defects, called "artefacts", in the images and a decrease of the spatial resolution.
An aim of the present invention, therefore, is to implement a method that enables correcting the defects resulting from deviations in the positions of the source and the detectors with respect to their theoretical position.
Another aim of the invention is to achieve a system implementing said method of correction.